On the Climate of Strathpeffer Spa. 55 
sunshine of the middle part of the day, and this must be 
much more than a sample or average of the entire day’s 
radiation. But since these facts apply equally or nearly 
equally to all stations, it may be permissible to use the 
fraction for duration in an expression for comparative 
intensity or value. Thus the fraction one-twelfth multiplied 
into 2°66, the mean daily value in zenith radiation, gives an 
expression 0°22, which although not the actual zenith value 
of the London winter’s day, is legitimately comparable with 
corresponding expressions similarly derived from the data 
proper to other localities. The corresponding expression for 
Strathpeffer Spa is 0°26 (one-fifth of 1:32); and we thus 
arrive at the result that the actual zenith value of radiation 
recorded at Strathpeffer during the winter is to that of 
London as 26 to 22. That is to say, the amount or value of 
solar energy in the form of light, heat, and chemical force 
actually transmitted by the atmosphere and received by the 
earth is just one-seventh more at the northern station. 
The importance of the atmosphere as a factor determining 
the amount of the solar radiation has been already insisted 
upon. There are, however, as yet no means of observing or 
recording the condition of this element at different times and 
places, There is no-standard of atmospheric purity or trans- 
parency, using that term in the wide sense above defined. 
Valuable data may be forthcoming from an examination of 
the sun’s rays, as they reach us under varying conditions of 
the air. The different elements of the solar radiation— 
luminous, thermal, actinic—are intercepted and absorbed 
more or less in all states of the atmosphere. It is required 
to assign this obstruction or absorption to the proper element 
or elements of the atmosphere by which it is occasioned, 
whether aqueous vapour (humidity), or solid or liquid matters 
in suspension. If it were possible so to measure and inter- 
pret the loss of the sun’s beams, we should at once have the 
desired criterion of the atmospheric state. But at present we 
have only general facts and inferences. It is obvious that 
in place of giving free passage, the atmosphere exacts a more 
or less heavy tribute, differing widely in amount in different 
places. And it is allowable to infer that the simple duration 
